Biosynthesis of polyunsaturated fatty acids (hereinafter abbreviated as “PUFA”) in humans occurs for two representative series, the ω3 and ω6 series (where ω represents the number of the carbon atom having the first double bond, counting from the methyl group end of the fatty acid), and in the case of ω6 fatty acids, for example, linoleic acid (18:2 ω6) is converted to γ-linolenic acid (18:3 ω6), dihomo-γ-linolenic acid (20:3 ω6), arachidonic acid (20:4 ω6) and 4,7,10,13,16-docosapentaenoic acid (22:5 ω6), by repeated desaturation and carbon chain elongation.
Similarly, in the case of ω3 fatty acids, α-linolenic acid (18:3 ω3) is converted to eicosapentaenoic acid (20:5 ω3), 7,10,13,16,19-docosapentaenoic acid (22:5 ω3) and 4,7,10,13,16,19-docosapentaenoic acid (22:6 ω3), by repeated desaturation and carbon chain lengthening. The ω3 PUFAs eicosapentaenoic acid (hereinafter, “EPA”) and docosapentaenoic acid (hereinafter, “DHA”) in particular are known to have numerous physiological functions including prophylactic effects against adult diseases such as atherosclerosis and thrombosis or anticancer effects, as well as learning reinforcement effects, and various attempts have been made to utilize them in pharmaceuticals and specific health foods. However, the physiological functions of PUFAs other than ω3 types (such as ω6 and ω9) have recently also been the subject of attention.
Arachidonic acid constitutes approximately 10% of the fatty acid components of vital organs such as the blood and liver (for example, the fatty acid compositional ratio of the phospholipids in human blood is 11% arachidonic acid, 1% eicosapentaenoic acid, 3% docosapentaenoic acid), and as a major structural component of cell membranes, it contributes to modulating membrane fluidity and performs various metabolic functions in the body, while also playing an important role as a direct precursor of prostaglandins. Recently the roles of arachidonic acid as a nursing infant nutrient and as a constituent fatty acid of endogenous cannabinoids which exhibit neurergic effects (2-arachidonoyl monoglycerol and anandamide) have been noted. Normally, ingestion of linoleic acid-rich foods leads to their conversion to arachidonic acid, but since the functions of the enzymes involved in its biosynthesis are reduced in patients with adult diseases or preliminary conditions as well as in infants and the elderly, such individuals tend to be deficient in arachidonic acid; it has therefore been desirable to provide means for its direct ingestion in the form of a constituent fatty acid of fats or oils (triglycerides).
Although fish oils are abundant sources of ω3 PUFAs such as EPA and DHA, ω6 PUFAs such as γ-linolenic acid, dihomo-γ-linolenic acid, arachidonic acid and 4,7,10,13,16-docosapentaenoic acid (22:5 ω6) are virtually unobtainable from traditional fat or oil sources, and therefore fats and oils comprising PUFAs as constituent fatty acids (hereinafter referred to as “PUFA-containing fats and oils”) obtained by fermentation of microorganisms are most commonly used at the current time. For example, methods have been proposed for obtaining fats and oils comprising arachidonic acid as a constituent fatty acid (hereinafter referred to as “arachidonic acid-containing fats and oils”) by culturing of various microorganisms capable of producing arachidonic acid-containing fats and oils.
It is known that fats and oils having a high proportion of arachidonic acid constituting the fatty acid portion (hereinafter referred to as “arachidonic acid-rich fats and oils”) can be obtained by using microorganisms belonging to the genus Mortierella 
(Japanese Unexamined Patent Publication SHO No. 63-44891, Japanese Unexamined Patent Publication SHO No. 63-12290). In recent years, one of the essential uses of arachidonic acid is in the field of nursing infant nutrition, for example, and specifically the use of arachidonic acid-containing fats and oils obtained by fermentation in modified milk has been introduced. New effects of arachidonic acid-containing fats and oils have also been demonstrated (Japanese Unexamined Patent Publication No. 2003-48831: Composition with prophylactic or ameliorative effect on symptoms and conditions associated with brain function impairment), and these are expected to be in high demand in the future.
Fats and oils obtained by culturing of Mortierella microorganisms largely consist of triglycerides (approximately 70% or greater) and phospholipids. The edible fats and oils are in the form of triglycerides, and for the purpose of the use described above, the original fats and oils produced by the cells (fats and oils obtained by extraction from cells, known as “crude oils”) are extracted from the cell biomass produced by culturing of the microorganisms, and then the crude oils are subjected to edible fat/oil refining steps (degumming, deoxidation, deodorization, decolorizing) to obtain refined fats and oils minus the phospholipids.
Since PUFA-containing fats and oils obtained by culturing of Mortierella microorganisms accumulate in hyphae, culturing must be carried out to a higher concentration to increase the yield of the PUFA-containing fats and oils per culture, for increased economy of the fat/oil production. The PUFA-containing fat and oil yield per culture is the product of the cell concentration and the PUFA-containing fat/oil content per cell, and it is therefore necessary to increase both the cell concentration and the PUFA-containing fat/oil content per cell. The cell concentration can be increased by raising the concentration of the medium nitrogen source, which is normally converted to cell components.
The PUFA-containing fat/oil content per cell can only be increased by satisfactorily controlling the cell morphology and by carrying out the culturing with an adequate oxygen supply. Methods reported for controlling the cell morphology include optimization of the medium salt composition (Japanese Domestic Re-publication No. 98/029558), while methods of supplying oxygen include pressurized culturing methods and oxygen enriched aerobic culturing methods (Japanese Unexamined Patent Publication HEI No. 06-153970). However, since these methods are affected by slight differences in the culturing conditions, it is not easy to ensure reproducibility of culturing and as a result, stable production output has not been possible to achieve.    Patent document 1: Japanese Unexamined Patent Publication SHO No. 63-44891    Patent document 2: Japanese Unexamined Patent Publication SHO No. 63-12290    Patent document 3: Japanese Unexamined Patent Publication No. 2003-48831    Patent document 4: Japanese Domestic Re-publication No. 98/029558    Patent document 5: Japanese Unexamined Patent Publication HEI No. 06-153970